Encode server name, LB policy name, and addresses in channel args.

include/grpc/impl/codegen/grpc_types.h

src/core/ext/client_config/lb_policy_factory.c

src/core/ext/client_config/lb_policy_factory.h

src/core/ext/resolver/dns/native/dns_resolver.c

src/core/ext/resolver/sockaddr/sockaddr_resolver.c

test/core/end2end/fake_resolver.c